Came across a fun little list from Mike Mendez, director of Big Ass Spider (2013), the Top 5 Elements of a Great Creature Feature. It’s probably a little more geared towards writer/director types but some of his advice can be applied to plain ole screenwriters.
If I could tack on one more thing, it would be the importance of a good build-up. Suspense and imagination are your best special effects -- especially on low-budget horror scripts. The tease of the creature is often more effective than the eventual reveal of a guy in a rubber suit or a fuzzy CG creation. I mean, sure, you could reveal the bat-winged, monkey-bird on page 10, but then what? Take a look at this:
The second I got a good look at the Creeper in Jeepers Creepers, the movie lost me a little bit. My rational brain could explain that it was simply a guy in a rubber mask. Cool design though...
While a lot of creature features leave very little to the imagination these days when it comes to marketing *cough*SyFy*cough*, the scripts themselves shouldn't be approached in the same manner.